Mood swings and depression

I've been having really bad mood swingsfor the past 2 days, and I don't know why. I'm not PMSing; my period's not supposed to come for another 2 weeks. I've been really stressed, but I feel like there has to be more to it. I'm alright one moment, then the next moment I hate myself and I feel really dumb, even though everyone thinks that I'm the smartest girl in our grade. Then I calm down and I'm okay again. Then I get really mad at everyone and everything. I don't know why, but I want to. Does anyone know what it could be. Also, how can I stop stressing. I can't get rid of the cause of it because I'm stressed about school, but I want to do something about it.